The present invention relates to compositions for use as thermal insulation or barriers in articles that are required to function under transient elevated temperature conditions, such as are experienced during a fire. Articles in which compositions according to the invention may be used include electrical and optical cables which have fire resistant properties, electrical fittings such as terminals and cable clips, and void-filling compounds which are required to act as fire barriers.
Various materials are used as thermal barriers in articles that have to withstand, or continue to function at, elevated temperatures, such as, for example, may occur during a fire. Such materials include, for example, certain metal oxides which have been fused to form a ceramic coating and inert minerals which remain in a stable solid state up to temperatures in excess of the highest temperature under which the article is required to perform. For example, magnesium oxide, which remains essentially inert at the temperatures achieved by a normal organic matter fuelled fire, may be used. The magnesium oxide may be contained within a metallic housing.
Polymer materials have been used. Some polymers, such as silicon rubber, are stable at temperatures up to around 300xc2x0 C. but above these temperatures decompose to form an insulating ash of silicon dioxide which then remains stable to higher temperatures. The silicon dioxide ash is however fragile and may fall away or fracture. Other polymers, such as polyamides, fuse at temperatures around 300-400 xc2x0 C. and tend to flow away with a resultant loss of insulation from part of the article where it is required.
In the electrical and building industries, articles may have to comply with the standards laid down by ISO 834 Part I which requires an article to remain functional when subjected to heating according to a specified time/temperature curve. In many cases the performance of materials under such conditions is compromised by the temperatures at which the component materials change state or react chemically. One class of materials that is used in various forms is the aluminosilicates. In general, as the silicate content of aluminosilicates decreases the fusion temperature increases. Pure alumina fuses at approximately 2,050xc2x0 C. whereas the naturally occurring muscovite form of aluminosilicate fuses at approximately 1,100xc2x0 C. In most fire situations, the temperature does not reach 1,100xc2x0 C. and therefore if muscovite forms part of the fire barrier system, it remains in the solid phase throughout the fire. Such a system is prone to mechanical failure because the barrier system is in a non-ductile state during the fire. If the muscovite is in particle form and held in place by other components of the barrier system which lose integrity as the temperature increases, it is liable to fall away. If the muscovite is in sintered form it is liable to be fractured due to movement caused by thermal expansion, mechanical shock and vibration experienced during the fire. Fracturing allows the fire front and hot gaseous products of combustion to penetrate the barrier system. In applications where the composition is used as a void-filling fire barrier, failure may enable combustion to penetrate and spread the fire, for example from one room into the next or from one floor to the next in a building. In electrical applications, failure of the fire barrier is likely to result in loss of electrical integrity.
There is need for a composition having thermal insulating properties, that is ductile or flexible at the elevated temperatures experienced during a fire and retains integrity so as to stay in place throughout the fire enabling it to continue to function as a thermal barrier.
According to the present invention in a first aspect, a composition for use as thermal insulation or barrier in articles that are required to function under transient elevated temperature conditions, comprises at least two components, a first of the two components, at temperatures within a first lower temperature range, being ductile or flexible, but undergoing a physical or chemical change at temperatures above the lower range but below an upper temperature limit required for fire resistance performance, a second of the two components being dispersed within the first component, and the first component being cohesive in the lower temperature range so as to retain the second component and to stay substantially in place, the second component undergoing a physical or chemical change in transition to a second upper temperature range, the second component being ductile or flexible in the upper temperature range, and being cohesive so as to stay substantially in place at temperatures in the upper temperature range, the lower limit of the upper temperature range being below the upper temperature limit for fire resistance performance.
In the compositions, the first and second components are mixed together. In the event of a fire, the first component or the reaction products of the first component are dispersed throughout the second component at the upper temperature range.
The change of state of the first component may be fusion or decomposition. The change of state of the second component may be fusion or decomposition.
The first component may comprise a polymer. In this context the term polymer is used to include the polymer itself and/or reagents that may be reacted together to form a polymer. The polymer may be a silicon rubber. Silicon rubber is stable at temperatures up to approximately 300xc2x0 C. but above these temperatures it decomposes to form an ash of silicon dioxide. The silicon dioxide ash is fragile and, on its own, would tend to fall away or fracture. Other polymer such as polyamides and polyesters fuse at temperatures around 300-400xc2x0 and, on their own, would tend to drip away thereby losing insulation from the article.
In the case where the first component is a polymer which fuses at temperatures above the first temperature range, the presence of the second component in the polymer may serve to increase the viscosity of the fused polymer and thereby enable it to stay in place at temperatures above, say 400xc2x0 C.
The second component may comprise a glass, a mixture of materials which when fused form a glass or a crystalline material. The second component fuses at a temperature below the maximum temperature which the article is required to withstand. The second component may start to fuse at temperatures above the melting or decomposition of the first component. Where the first component is a material that decomposes to a solid phase residue, the residue may be dispersed within the fused second component and serves to increase its viscosity helping it to remain in place as the temperature increases.
The composition may include a third component which remains stable in a solid phase throughout the upper temperature range so as to increase the viscosity of the fused second component. The third component may be included where there is no solid residue as a result of decomposition of the first component or it may be included where there is a solid residue in order to increase the viscosity of the fused second component further.
The third component may be an oxide of aluminum, silicon or magnesium or a combination of such oxides, such as an aluminosilicate. The third component may remain in a solid state at temperatures up to and above the upper temperature limit required for fire resistance performance, for example 1000xc2x0 C., it may fuse at temperatures below this upper temperature limit but above the lower limit of the upper temperature range. In this way, the third component may form a ductile system which provides cohesion in a second upper temperature range. For example, aluminosilicate may be treated to reduce its fusion temperature to lie within the upper temperature range and below the upper limit at which the composition is required to perform.
As stated above, aluminosilicates have temperatures of fusion lying in the range from 1,100xc2x0 C. to 2,050xc2x0 C. We have found that by treating aluminosilicates with other materials, the temperature of fusion can be lowered to lie within the range of temperatures likely to be experienced in the event of fire. Typically the materials comprise metal oxides or precursors to metal oxides. Preferred metal oxides are alkali metal oxides. We have found that by using sodium oxide in the form of sodium carbonate significant reductions in the fusion temperature of aluminosilicates can be achieved. For example, by treating mica in the form of muscovite with sodium carbonate, the temperatures of fusion can be reduced from around 1,100xc2x0 C. to around 850xc2x0 C. As well as sodium oxide, lithium oxide or potassium oxide or combinations of sodium, lithium and potassium oxides may be used. Using such combinations the temperatures of fusion may be lowered to around 500xc2x0 C. An aluminosilicate treated in this manner may form the third component, or the second component referred to above.
A problem that may be encountered when the second component is made of materials containing sodium or potassium is that the material may be naturally deliquescent making it unsuitable for some applications.
Preferably the composition is non-deliquescent. We have found that suitable materials for the second and/or third components can be prepared using other materials, for example lead oxide containing glasses or the raw materials for such glasses. Other metallic oxides may be used.
The second component may be in the form of glass frits. The use of glass frits, however, has a disadvantage in that it adds considerably to the costs of the composition. Heating the raw ingredients to form the glass and then fitting and grinding the glass particles so that it can be incorporated into the first component involves a considerable amount of energy and several processing stages. The glass frits therefore add considerably to the cost of the composition. We have found that satisfactory second components can be formed by a mixture of the raw ingredients for a glass. Thus, according to a preferred form of the invention the second component comprises a mixture of discrete particles of different oxides which on heating form, in situ, a glass. A possible disadvantage with a second component in this form is that there may be a delay in melting due to the time taken for the ingredients to mix to form the glass and the fusion temperature of the ingredients may be higher than for the glass. On the other hand, the heat required to fuse the ingredients may have a beneficial effect in taking energy away from the fire.
The preferred materials are materials which, on fusion, form a low melting point glass, such as borate glass, phospate glass, sulphate glass, electronic glass. When the first component is a silicone polymer which decomposes to form silicon dioxide, and/or where silicon dioxide is added to the composition some of the silicon dioxide will mix with the fused glass to form a viscous paste, some may dissolve in the fused low melting point glass and thereby making a higher melting point glass and thus increasing its viscosity.
According to the present invention in a second aspect, a composition for use in the manufacture of thermally insulating fire resistant materials includes a polymer and a fusible oxide material mixed or treated with a metal oxide or metal oxide precursor or a combination of metal oxides or metal oxide precursors to form a composition which fuses at a temperature below the temperature likely to be experienced in the event of fire. Typically the temperature of fusion is lowered to be a temperature below 1,000xc2x0 C. The fusible oxide may be an aluminosilicate which has a fusion temperature above 1000xc2x0 C. before treatment. For lower temperature ranges a borate may be used, for example zinc borate. The metal oxide may be an alkali metal oxide, for example sodium oxide or lithium oxide or potassium oxide, or a combination of oxides from the group of sodium, lithium and potassium oxides, or it may be lead oxide, antimony trioxide, or precursors of these materials or mixtures of these oxides or precursors.
The amount of lowering of the temperature of fusion is dependant upon the proportions of metal oxide and fusible oxide in the mixture.
Preferably the fusion takes place over a range of temperature which enables the composition to maintain structural integrity over this range of temperatures. One advantage of forming the composition that fuses as a mixture of metal oxides or precursors is that this increases the range of temperature over which fusion takes place.
Preferably the range of temperature is between 450xc2x0 C. and 1210xc2x0 C.
As indicated above, the composition may comprise a mixture of the fusible oxide and the metal oxide or metal oxide precursor, or it may comprise the fusible oxide pre-treated by already being heated with the metal oxide so as to react with it to form the composition of lowered fusion temperature.
In order to improve the performance of the composition at higher temperatures, refractory materials, that is materials which remain solid and stable at temperatures above the temperature likely to be encountered in the event of a fire, may be added to the composition. By adding such materials, the viscosity of the fused crystalline material is increased so as to prevent the fused crystalline material flowing away from the article. The inclusion of refractory materials may also serve to stabilise the volume of the composition.
According to the present invention in a further aspect, a composition comprises a refractory oxide and a glassy material modified so that structural integrity of the composition is maintained over a wide range of temperatures, the modification consisting of adapting the fusion characteristics of the glassy material and the refractory oxide so that a viscous liquid phase is maintained during various stages of a fire, and structural integrity is maintained through liquid-solid phase bonding. In this way a more effective fire barrier material can be produced.
Suitable refractory oxides include silicon oxide, aluminium oxide and aluminosilicates.
The fusible oxide may have a fusing or softening point in the range 400xc2x0 to 850xc2x0. Lower softening points may be achieved by the addition of other materials.
The modified refractory oxide may be used with other materials to form systems that may be combined with a polymeric host material to provide flexible materials or void-filling compounds. These compounds can be designed to be electrically conductive, semiconductive, insulative or flame retardant, depending upon the application. Alternatively, the material may be used on its own.
Since the conditions experienced during a fire are transitory, there will be a temperature gradient throughout the article so that different parts of the article will be at different temperatures at any time. For example, when the article is an electronic cable, one side of the cable may be closer to the fire than the other and so the temperature on the outside of the cable closest the fire may be much higher than the interior of the cable on the side away from the fire. For this reason, it may not be necessary to provide a system which is flexible and provides integrity at all temperatures. Provided that at all times there are some appropriate parts of the article that are at temperatures at which one or other of the components of the system is providing the necessary integrity, there may be other parts of the article at temperatures at which neither component is in the appropriate phase.
The present invention also contemplates a composition comprising any two or more of the following elements:
a polymer to provide integrity and ductility at ambient temperatures and throughout a lower elevated rate of temperatures (the range may be, for example, up to 300xc2x0 C.)
a glass or fusible oxide component to provide integrity and ductility at a higher temperature range (the range may lie between, for example, 450xc2x0 C. and for example 850xc2x0 C.)
a modified refractory oxide or precursors to a modified refractory oxide to provide integrity and ductility in a still higher temperature range (the range may lie between, for example, 850xc2x0 C. and, for example 1,000xc2x0 C.), and
a refractory material which is stable and remains solid at all temperature conditions experienced during a fire (for example up to 1,000xc2x0 C.).
The components that are in the solid phase at any particular temperature when other components are in the fused phase may serve to increase the viscosity of the composition and thus improve the integrity of the system at that temperature.
Where the polymer is of a type that decomposes at temperature above the lower range to form a solid residue, the products of decomposition may form the refractory material that is stable at all temperatures or it may be additional to other refractory materials that are stable at all temperatures.
A system may comprise all four components. In such a system, as the host polymer decomposes, the glass or fusible oxide particles fuse to begin the process of substituting a ductile glass for the polymeric phase. As the glass or fusible oxide melts, it binds to or dissolves or bonds to the refractory oxides that form part of the original composition (that is both the stable refractory material and the modified refractory material) and the other refractory materials that may be present as a product of the decomposition of a host polymer. This action increases the liquid phase viscosity.
As the temperature increases, in the still higher temperature range, physical and/or chemical interaction or reaction occurs between the modifying agent, for example the alkali metal oxides, and the refractory oxides to form species capable of fusing at, for example, approximately 850xc2x0 C. This reaction progresses with some of the as yet unmodified refractory oxide from the solid phase and further with the refractory oxide in a fused phase that is dissolved into the liquid phase formed by the glass or fusible oxide melting. Both the liquid phase formed at the point at which polymer decomposition occurs and the fused modified refractory oxide bond to the stable refractory materials that are present as having been selected so as not to fuse within the temperature range which the article is expected to survive in the event of fire. These non-fusing refractory materials increase the viscosity of the fused refractory oxide and may serve to stabilise its volume.
Minor degrees of absorption of the non-fusing refractory material into the liquid phase formed by fused glass or oxide and the fused modified refractory oxide phases may occur. This action serves to provide a stable matrix even at very high temperatures.
Glasses are non-crystalline solids and therefore have non-uniform molecular bonding. This gives rise to a non-specific broad melting or softening point. The rate of change of viscosity with respect to temperature, although composition dependent, is relatively slow when compared to crystalline solids. By using a mixture of different types of glasses, or a mixture of fusible oxides that form, on heating, a glassy material, compositions can be provided that have an intermediate melting characteristic. By using glasses that devitrify at higher temperatures, interaction between the crystals formed on divitrification and the remaining liquid phase allows the viscosity to be maintained at temperatures at which the liquid phase would otherwise be too fluid to maintain integrity. Thus the upper end of the temperature range at which the composition is effective may be extended.
In applications where electrical insulating properties are important, lead oxide containing glasses are particularly effective. Electronic glasses or glasses that contain species capable of neutralising ionic fragments of the mobile phase may also be used.
For glasses that are slightly electrically conductive in nature, materials with increased alkaline metal content and lower temperatures of fusion are suitable. Chalcogenides and metglas can be used where electrical conductivity is required.
For lower temperature fusing glasses, phosphate, sulphate and boric oxide glasses with various modifiers and stabilisers are typical.
Borate glasses or fusible oxides that form borate glasses are particularly preferred. We have found that by using a mixture containing lead oxide and a borate, a satisfactory composition can be achieved. The mixture may also contain antimony trioxide.
As indicated above, there can be considerable cost savings if the fusible oxide component is present in the composition as a mixture of the raw ingredients for a glassy material rather than as the glass in frit form.
For the formation of a refractory oxide with a lowered fusion temperature, a combination of refractory oxide with alkali metal oxide donating fluxes are included in the composition. Preferably the alkali metal oxide is sodium or lithium oxide or a combination of sodium, lithium and possibly potassium. By using combinations of alkali metal oxides, fusion temperatures below 850xc2x0 C. can be achieved, even as low as 400-500xc2x0 C. The flux acts to break up the covalently bonded structure of the refractory oxide. This is important as bond strengths of the refractory oxide are typically 60-110 kcal/mol and are directly related to the temperature at which melting occurs. Therefore, within limits, the quantity of alkali metal oxide added determines the fusion temperature of the modified refractory oxide.
Depending upon the application to which the system is to be employed, the modified refractory oxide can be present in either one of two distinct forms. Where there are no overriding electrical requirements, the system can be applied with the component in an untreated state, that is to say the as yet unmodified refractory material and the modifying agent are co-mixed but un-reacted. This has the advantages of eliminating one step from the manufacturing process and may provide an additional enothermic flame retardant effect through the liberation of non-combustible gases (for example, nitrogen, water or carbon dioxide). In other circumstances, such as where there is a need for electrical insulating properties or the elimination of evolved gasses, the modified refractory oxide may be pre-prepared, that is the components of the system un-modified oxide and modifying agent are fused, fritted and ground before being incorporated in the composition that is applied to form the article. In this way, the gas liberating reactions completed prior to the incorporation to the system into a host polymer or, when using the composition as a coating composition, without further adaptation. The pre-prepared modified oxide may also have advantages in certain applications in that the physical and chemical changes that are taking place as the composition undergoes the temperature change of a fire are simplified which may enable the thermal transitions to be rationalised to provide distinctive ends to the temperature ranges at which the various components of a system perform.
As indicated above, a composition according to the invention does not necessarily include all four components. It may consist merely of the polymer and the fusible oxide or glass, or the polymer, the fusible oxide or glass and the refractory material.
According to the present invention in another aspect, a method for producing a crystalline material with a fusion temperature below the maximum temperature that is likely to be experienced in a fire, for example 1,000xc2x0 C., comprises adding an alkali metal oxide, or a mixture of alkali metal oxides and precursors to such oxides to a refractory oxide, for example, aluminum oxide, silicon oxide, magnesium oxide and compounds of these oxides, such as for example aluminosilicates, and fusing the resultant composition.
According to the present invention in another aspect, the present invention provides a composition for use in the manufacture of thermally insulating fire resistant materials including a polymer and a mixture of discrete particles of different oxides which, on heating, fuses to form in situ a low temperature fusing glass.
Preferably the polymer is a silicone polymer that decomposes to form oxides of silicon upon heating.
Preferably the mixture fuses at a temperature in the range 450xc2x0 C. to 850xc2x0 C.
Preferably the mixture includes a borate, for example zinc borate.
Preferably the mixture includes a metal oxide, preferably lead oxide or antimony oxide or a mixture of lead and antimony oxides.
The composition may also include a refractory material, for example silicon dioxide.
The compositions described above may be used for making a wide variety of articles, by moulding extrusions or other processes.
According to the present invention in a further aspect there is provided an article that is required to perform under transitory elevated temperatures, including thermal insulation comprising any of the compositions described above.
The article may be an electric cable comprising one or more electric conductors. The thermal insulation may constitute the electric insulating material or one or more of the layers of electric insulating material or it may comprise a separate layer. The article may be an electric cable clip or other accessory for an electric cable, or it may be an electrical component.